The invention relates generally to evaluation of alternator operation and, more specifically, to fault detection of an alternator based on the frequency of an alternator output signal.
An alternator converts mechanical motions into alternating current (AC) by electromagnetic induction. The alternating current is then passed through a rectifier assembly, such as a full-wave rectifier bridge comprising diodes, to convert the AC into DC to power other electrical systems. For example, an alternator in an automotive vehicle is driven by the engine to power the vehicle""s electrical system, such as for charging battery, powering headlights, and the like.
The output of an alternator, even though rectified, is not perfectly smooth. The waveform of an alternator output, after rectification, is generally a low-magnitude ripple riding on a DC component. An example of an alternator output signal of a three-phase alternator, after rectification, is shown in FIG. 1.
The rectifier assembly used in alternators comprises diodes. If one or more of the diodes fail, such as open or short-circuited, the alternator output waveform becomes considerably distorted. FIGS. 2a-2e show examples of alternator output waveforms when one or more diodes in the rectifier assembly fail. FIG. 2a is the output waveform of a normal alternator. FIG. 2b shows a waveform of an alternator having a short-circuited rectifying diode, FIG. 2c shows a waveform of an alternator having two short-circuited rectifying diodes, FIG. 2d shows a waveform of an alternator having one diode open-circuited, and FIG. 2e shows a waveform of an alternator having two diodes open. In any case, the distorted alternator output waveform is likely to damage the electrical system powered by the alternator. Accordingly, knowing whether an alternator and its rectifier assembly operate normally is important to alternator testing.
One way the operating condition of the diodes is determined is by disassembling the rectifier assembly and individually measuring each diode. This approach, however, is time-consuming and inefficient. Some alternator testers determine the health of an alternator by detecting the ripple amplitude of alternator output signals. The distorted ripple waveforms, as illustrated in FIGS. 2b to 2e, have amplitude larger than normal ripple waveforms shown in FIG. 2a. Some testers, therefore, pass the alternator output signal through a comparator having a preset threshold voltage A (See FIG. 3a). The comparator detects crossing of the threshold voltage by the ripple. The comparator outputs a logic state xe2x80x9c1xe2x80x9d when the ripple voltage is above the threshold voltage, and a logic state xe2x80x9c0xe2x80x9d when the ripple voltage is lower than the threshold voltage. For a normal output waveform, the ripple amplitude is always above threshold voltage A. Therefore, the output of the comparator is logic state xe2x80x9c1.xe2x80x9d Conversely, if the rectifier assembly is defective, the ripple waveforms will be similar to those illustrated in FIGS. 2b to 2e and the valley voltage of the ripple will be lower than the threshold voltage A. A logic state xe2x80x9c0xe2x80x9d will appear on the output of the comparator indicating a defective alternator.
This approach, however, causes problems. The DC component of the alternator output, for reasons such as change of engine rotational speed, tends to float between different DC levels and thus the ripple voltage fluctuates accordingly. Since the threshold voltage is a fixed value, the level of the fluctuating waveform may drop below the threshold voltage (See FIG. 3b). Consequently, the comparator will indicate a defective alternator even if the rectifier assembly works well. Conversely, when the DC level rises above the threshold voltage, the output of the comparator is at logic xe2x80x9c1xe2x80x9d despite that one or more diodes may be defective. Under these circumstances, using ripple amplitudes to determine the health of the alternator is likely to produce errors.
Even if the engine rotational speed is stabilized during the test, the method using ripple amplitudes still causes problems. For instance, some vehicles are equipped with batteries located remotely from the alternator and connected to the alternator with long wires. The conducting wires spanning between the battery and the alternator have different impedance from those close to the alternator. This difference in impedance tend to affect the DC level of the alternator output signal and thus, as described above, creates incorrect test result.
Accordingly, there exists a need to efficiently determine the health of an alternator. There is also a need for determining the health of an alternator accurately. There is another need for evaluating the health of an alternator from the alternator output signal. An additional need exists for correctly determining the health of an alternator irrespective of the location of the battery. Still another need exists for an adaptive threshold to produce a correct waveform representing the frequency component of the alternator output signal.
The method according to the present invention comprises detecting a frequency component of the alternator output signal, comparing the frequency component of the alternator output signal with a threshold frequency, and evaluating the operation of the alternator based on a result of the comparison.
The system for evaluating the operation of an alternator per the invention comprises a terminal for receiving an alternator output signal representative of an output of the alternator, a frequency detection device for detecting a frequency component of the alternator output signal, a controller for comparing the frequency component of the alternator output signal to a threshold frequency and generating an indication signal based on a comparison result, and an indication device responsive to the content of the indication signal for indicating the operation of the alternator.
According to one aspect of the invention, the frequency detection device may comprise, for example, a threshold device for generating a reference threshold and a comparator for comparing the level of the alternator output signal with the reference threshold. The comparator produces a frequency signal representative of the frequency component of the alternator output signal. According to one aspect of the invention, the reference threshold may be generated based on the level of the alternator output signal according to a predetermined rule. The reference threshold may be a value between a peak signal level and a valley signal level of the alternator output signal. For example, the reference threshold may be the average of the peak signal level and the valley signal level.
According to an embodiment of the invention, a system for evaluating the operation of an alternator comprises a terminal for receiving an alternator output signal representative of an output of the alternator, an adaptive threshold device for generating a reference threshold based on the level of the alternator output signal according to a predetermined rule, and a comparator for comparing the level of the alternator output signal with the reference threshold and generating a frequency signal indicating the frequency component of the alternator output signal based on the comparison result. A controller, responsive to the frequency signal, compares the frequency component of the alternator output signal to a threshold frequency and generates an indication signal representative of the operation of the alternator based on the comparison result of the frequency component and the threshold frequency. An indication device responsive to the content of the indication signal indicates the operation of the alternator.
The method and system of the invention, using the frequency component of the alternator output signal to evaluate the health of an alternator, provide an efficient and precise indication of the operation of the alternator. False diagnosis is reduced, even as the DC level of the alternator output signal fluctuates.
Still other advantages and novel features of the present invention will be apparent from the following detailed description, simply by way of illustration of the invention and not limitation. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.